The fabrication of electronical systems on flexible substrates, such as plastic foils, paper or metal, is becoming more and more important. The following may be designated as future applications: flexible displays, flexible solar cells, flexible luminous areas, and, in general, electrical circuits which are easily deformable due to the flexibility of the substrate and, thus, may optimally fill predetermined housing volumes in a space-saving manner.
Printed circuits on the basis of organic materials, such as semiconductive polymers, polymer conductor trace materials, polymer dielectrics etc. are one current development focus in the area of flexible electronics. It is the printing technology in particular that offers a very cost-effective processing technique. In connection with flexible substrates, continuous roll-to-roll production techniques may be utilized for this purpose. During the development phase of new methods and materials for flexible electronics, however, continuous roll-to-roll processes are too expensive. Here, it would be advantageous to be able to process the flexible substrates also as single sheets in plants and process systems. This would also open up the possibility of using already existing production plants for flexible substrates, too.
However, handling single flexible substrates is difficult. For coating processes with liquid media, drying processes in the oven or in a lithographic patterning, the foils must be flat-fixed on a platen and additionally be capable of being processed with automatic handling systems.
If mounted onto rigid substrates, single flexible foils may be processed in coating plants. A simple variant for this purpose is to stick the edges of a foil sheet on a rigid carrier plate with slightly larger dimensions, by means of sticky tape. However, this is associated with a significant amount of manual work. Also, stripping the sticky tapes after temperature steps is often difficult or associated with significant remains. Additionally, in the case of drying processes on hotplates, no uniform, planar heat contact is present; air bubbles and cambers of the foils prevent a really flat resting of the substrate sheet. A non-uniform heat contact and a corrugated form of the substrate inevitably lead to non-uniform layer thicknesses, which, in the case of conducting or semiconducting layers, leads to highly varying and, thus, useless electronical properties.
In principle, a full-area sticking of the foil to the carrier substrate is possible; however, this is associated with even larger processing expenses. For this reasons, re-detachable sticking techniques do not present any cost-effective and practicable solution. Alternatively, the foils may be fixed on perforated vacuum suction plates. Then, however, the automatical charging and discharging of the corrugated foils is still not solved in a reasonable manner.
Electrostatic holding techniques for thin substrates are known, too, wherein unipolar and bipolar holders, or carriers, are distinguished. In the case of the unipolar carrier, the carrier plate comprises only one junction electrode and, accordingly, only carries charges of one polarity (unipolar). With a bipolar carrier, a carrier plate comprising at least two electrically separated junction electrodes charged with opposite polarities is utilized. A device for a unipolar carrier is described in JP 59132139, and a corresponding method for holding is indicated in DE 10235814 B3. This holding is suitable for electrically conducting substrates, wherein a layer with a high dielectrical constant is put between the substrate and the carrier plate, so that fixing of the substrate is made by dielectrical polarization. Furthermore, a mobile holder based on a bipolar carrier is described in WO 02111184 A1. This holding is provided for wafers.
These electrostatic methods have the benefits of a good planar contact pressure, an easy detachability of the electrostatic carrier, and an automatization without any problems. It has turned out, however, the foils such as PET (PET=polyethylene terephthalate, polyester) or polyimide comprise holding forces which are too weak towards electrostatic fields, and for this reason, these methods, or devices, do not provide any reliable holding for these materials. Due to the increasing importance of these materials, the current situation is not satisfactory.